This invention relates to the control of refrigeration-coupled thermal energy storage systems for buildings. More particularly, the invention relates to control systems and methodologies which allow building heating, cooling, and water heating loads to be satisfied from refrigeration-coupled thermal energy storage systems with minimal electrical energy input to drive the refrigeration compressor, liquid pumps, and fans.
Electric utilities increasingly pursue strategies to level the overall electrical use pattern among their customers, as a means to improve their economics. Cooling and heating loads in buildings comprise significant elements of overall electrical energy use. Refrigeration-coupled thermal energy storage (RCTES) systems provide a means for accomplishing significant shifts in electrical use patterns for building cooling and heating systems. RCTES systems allow decoupling of compressor operation from building loads; instead of operating when a building thermostat demands heating or cooling, the RCTES compressor runs to accomplish a desired thermal storage condition sufficient to satisfy subsequent thermostat demands from the building.
To foster introduction of RCTES systems, many electric utilities offer "time-of-use" rates under which customer "per kilowatt-hour" energy charges are increased for on-peak use and decreased for off-peak use. Many utilities also offer initial incentive payments for installation of RCTES systems. In response to these utility load-shift programs, both single function (cooling or heating) and multiple function (cooling and heating and/or domestic water heating) RCTES systems are in use for both commercial and residential applications. U.S. Pat. Nos. 4,242,873, 4,270,518, 4,030,312, 4,242,872, 4,392,359, 4,302,942, 4,256,475, 4,246,956, 4,336,692, 4,279,359, 4,011,731, 4,645,908, 4,685,307 and 4,693,089 disclose various RCTES systems. U.S. Pat. No. 4,693,089 describes the RCTES system for which the optimal control system disclosed herein was developed.
Since a major goal of RCTES systems is to cause compressor operation during off-peak hours rather than in response to building thermostats, compressor control components in addition to the building thermostat are required. One simple RCTES control strategy initiates compressor operation at the end of each on-peak period, maintaining operation until either the off-peak period ends or a target storage condition is achieved (where the target condition is adequate to satisfy loads with extreme anticipated seasonal weather conditions). However, this simple strategy usually results in reduced operating efficiency because normal weather conditions allow lower target storage conditions than are required for extremes. Efficiency is reduced because heat pump efficiency is decreased and storage container thermal losses are increased by colder summer and warmer winter average storage temperatures.
Modern electronics offer potential for more sophisticated control systems designed to increase RCTES efficiency by regularly computing new storage targets based on current weather and storage conditions. Few of the prior systems disclose advanced control strategies. U.S. Pat. No. 4,270,518 presents one such strategy, for heating season operation of a solar-boosted RCTES system, but the heating-mode controller does not consider a time-of-use utility schedule. U.S. Pat. Nos. 4,645,908 and 4,685,307 disclose considerable detail on a residential RCTES system, but the system does not automatically vary storage targets with outdoor weather conditions; much of the description relates to monitoring capabilities designed to limit overall on-peak house energy consumption.
Prior systems also lack other desirable control capabilities for RCTES systems. For potential "packaged system" cooling storage applications, it is desirable to limit storage volume requirements by utilizing a liquid/solid phase change (usually via freezing and thawing of ice). In such phase change storage systems, improved system control requires frequent monitoring of ice volume. A reliable and accurate system for monitoring ice fraction is necessary for accurate system control. It is also desirable for both cooling and heating RCTES systems to modify storage targets based on their recent history. For example, a system which for several daily cooling cycles ends the on-peak period with large ice fractions may learn that its targets are too high, reducing subsequent targets to increase efficiency.
Combined cooling/heating RCTES systems must reverse the storage condition from hot to cold and back at least once per year. The prior systems fail to provide automatic reversal controls, yet manual reversals may adversely affect operating costs and/or occupant comfort. Combined systems sized with adequate ice storage for fully off-peak cooling operation may require some on-peak heating operation to avoid use of inefficient resistance heating. Intelligent controls are required to allow, yet minimize, the on-peak heating season compressor operation. In most winter climates, a control strategy is also necessary to defrost outdoor heat exchangers used as heat sources during the heating season. Defrost controls are generally known, but improvements are possible to reduce defrost energy consumption.
In RCTES systems which provide full domestic water heating in addition to space conditioning, control logic is necessary to determine priority between space conditioning and water heating functions, and to bias "domestic water only" cycles toward off-peak operation. The prior systems do not consider control requirements to accomplish these tasks. Also, high ambient summer temperatures endanger the compressor when an outdoor heat exchanger is used as a heat source during domestic water cycles; special controls are needed to protect operating components.